This invention relates to expression of a fused protein, more specifically to a fused DNA sequence including a DNA sequence coding a heat-resistant protein, a fused protein expressed by said fused DNA sequence, and a method for expressing said fused protein.
Progress in genetic engineering has enabled analysis of a protein which has been purified from a natural substance, at a genetic level and artificial amplification of a desired protein (Itakura et al., Science, vol. 198, p. 1056 (1977)). By application of a DNA sequence to which thio-redoxin (hereinafter referred to as xe2x80x9cTRXxe2x80x9d in the specification) (International Provisional Patent Publication No. 507209/1993) or glutathione-S-transferase (hereinafter referred to as xe2x80x9cGSTxe2x80x9d in the specification) (International Provisional Patent Publication No. 503441/1989) which has been invented thereafter is fused, even a protein which is inherently expressed with difficulty can be expressed, and a technique of expressing a fused protein has been used widely.
TRX and GST can be applied to fusion and expression of various proteins which are expressed with difficulty, but even in GST which has been essentially used for the purpose of expressing a soluble fused protein, a fused protein becomes insoluble depending on a protein to be fused so that productivity is lowered, or a fused protein to which TRX is fused may have a problem that a nonspecific reaction is liable to occur. Therefore, it has been desired to provide a fused protein having further excellent operatability and productivity.
Thus, an object of the present invention is to provide a novel fused DNA sequence having excellent operatability and productivity for expressing a desired protein or peptide, a fused protein expressed from said fused DNA sequence, and a method for expressing the fused protein using said fused DNA sequence.
The present inventors have studied intensively in order to solve the problems in the art and consequently found that when a DNA sequence coding a selected protein or peptide and a DNA sequence coding a heat-resistant protein are fused directly or indirectly and a fused protein is expressed from the resulting fused DNA sequence, the productivity of the desired protein or peptide is raised, and said fused protein has heat resistance to make a purification step simple and easy, to accomplish the present invention.
That is, the present invention relates to a fused DNA sequence comprising a DNA sequence coding a heat-resistant protein or peptide, fused directly or indirectly to a DNA sequence coding a selected protein or peptide, a fused protein expressed by said fused DNA sequence, and a method for expressing the fused protein using said fuse-DNA sequence.
The fused protein of the present invention has high solubility and can maintain even heat resistance derived from heat-resistant protein genes. Because of such a characteristic of the fused protein, when the fused protein is purified, unnecessary substances can be removed simply and easily by heat treatment so that the fused protein can be obtained with good yield.
In the case of TRX derived from Escherichia coli and GST derived from Schistosoma japonicum, which have been widely used as a fused protein, Escherichia coli and Schistosoma japonicum can live in bodies of mammals and other creatures so that when a fused protein using TRX or GST is used as an antigen of an immunoreaction, a nonspecific reaction due to Escherichia coli or Schistosoma japonicum might be caused. To the contrary, the great characteristic of the fused protein of the present invention resides in that a heat-resistant protein derived from a thermophilic bacterium which cannot live in living bodies of mammals and other creatures is used so that even when the fused protein of the present invention is used as an antigen of an immunoreaction, a nonspecific reaction derived from the fused protein is caused with difficulty.